The present disclosure relates to a fixing device for applying a fixing processing to a sheet carrying a transferred toner image, and an image forming apparatus incorporated with the fixing device.
A fixing device in an image forming apparatus is provided with a fixing roller and a pressing roller, and a toner image is fixed onto a sheet by allowing the sheet carrying the transferred toner image to pass a fixing nip portion formed by the fixing roller and the pressing roller for heating and pressurization. Generally, the fixing device is configured into a unit, and is detachably mounted on an image forming apparatus main body for maintenance service and the like. The fixing roller and the pressing roller are accommodated in a housing of the unit. Further, normally, a pair of conveyance rollers is disposed downstream of the fixing nip portion for feeding a sheet to a sheet discharge unit of the image forming apparatus main body, and the conveyance roller pair is also supported by the housing.
A sheet may be jammed in the housing of the fixing device. It is highly likely that a sheet may be jammed at a position between the fixing nip portion and the conveyance roller pair. Conventionally, there has been known a fixing device configured in such a manner that a part of a housing is formed into an openable and closable cover member so that a user is accessible to the fixing nip portion from the outside of the apparatus main body for allowing the user to remove the jammed sheet. In the fixing device, one of the conveyance roller pair is supported by the cover member. When the cover member is opened with respect to the housing for removing a jammed sheet, the fixing nip portion is exposed and at the same time, the nip portion of the conveyance roller pair is divided. Thus the user is allowed to easily remove the jammed sheet by opening the cover member.
A certain nipping pressure is required in the nip portion of the conveyance roller pair for giving a sheet conveying force. Further, generally, a torsion spring is mounted at a pivot portion of the cover member with respect to the housing to urge the cover member in a direction of returning the cover member from an opening posture that the cover member is opened with respect to the housing to a closing posture that the cover member is closed with respect to the housing. In the conventional fixing device, the nipping pressure of the conveyance roller pair is generated, utilizing an urging force of the torsion spring.
In the structure as described above, the nipping pressure of the conveyance roller pair may be unstable. One of the reasons of the drawback is that the torsion spring is an urging member disposed for the purpose of returning the cover member to the original posture, and therefore, it is difficult to generate a stable nipping pressure in the axis direction of the conveyance roller pair without variation. Further, the torsion spring has two functions i.e. a function of returning the cover member from an opening posture to a closing posture, and a function of generating a nipping pressure in the conveyance roller pair when the cover member is in the closing posture. In view of the above, it is necessary to use a torsion spring having a relatively large urging force. Consequently, an unduly large urging force may be applied to the cover member, and large impact noise may be generated resulting from such an unduly large urging force, when the cover member is closed. In a worse case, a part of the cover member may be damaged or broken.
In view of the above drawbacks, it is an object of the present disclosure to provide a fixing device having a cover member, which is capable of stably generating a nipping pressure of a conveyance roller pair, while preventing generation of impact noise.